The present invention relates to an electrical heating in a motor vehicle and more particularly, to a device for and a method of applying voltage to a heating resistor in a motor vehicle having a power generator and a power supply network connected across terminals of a battery.
To improve electrical power supply to a heating resistor, for example to a heated front pane of a motor vehicle, different possibilities are known which achieve a high heating efficiency.
For example, it is known to connect the power generator either to the power supply network whereby the output voltage of the generator is regulated to the nominal voltage value of the network, or the generator is connected to power supply of the heating resistor whereby in order to achieve a higher heating efficiency the generator is regulated to deliver an increased output voltage.
Since the power generator when connected to the heating resistor, serves exclusively for energizing the heating resistor, a high heating efficiency is provided even at a low rotary speed of the generator. Nevertheless, since during the heating operation both the battery and other electrical consuming devices have to be disconnected from the power generator, the power supply to consuming devices of the power supply network is effected by the battery only. The battery power supply however is possible only for a relatively short time before the battery is excessively discharged. Therefore, this solution can be provided only during the standstill of the motor vehicle when it is possible to minimize the energization of the consuming devices to save the charge of the battery.
The heating operation during the running of the motor vehicle would be possible when the output voltage of the generator be limited to the nominal voltage of the power supply network; this however would lead to a lower efficiency of the heating resistor.
A device of this kind for heating for a short time with an increased voltage and for a longer time with a nominal voltage of the power supply network is known from the DE-OS 27 05 049. In this prior art device the heating element is connected either directly to the output of the power generator and supplied with voltage which is distinctly higher than the nominal voltage of the power supply network whereby the power supply network with the battery are disconnected from the power generator. As mentioned above, this connection can be maintained only for a short time because the battery would become excessively discharged. For energizing the heating resistor for a longer period of time the prior art device provides a switch-over mechanism for connecting the heating resistor and the battery directly to the output of the power generator and during this connection the output voltage of the generator is limited to the nominal voltage of the power supply network.
A possibility to feed the heating resistor at an increased voltage with simultaneous feeding of the other electrical consuming devices at a nominal voltage of the power supply network is known from the DE-PS 38 01 478. In this embodiment, the heating element is directly connected to the output of the power generator and supplied with an increased generator voltage.
The nominal power supply voltage for the remaining consuming devices is derived from the increased voltage by means of a converter. However, an adequate power supply due to the requisite design of the converter for sufficient load currents can be achieved only with difficulties and, in addition, is very costly.
Furthermore, from the DE-PS 38 01 478 it is known to connect a heating resistor in series with the power supply network and during the non-heated operational condition of the vehicle, the heating resistor is short circuited.
By means of such a series connection the generator can energize the consuming devices up to a certain voltage level even during the heating mode of operation. Therefore, a high power heating is made possible both during the stillstand condition as well as during the running condition of the motor vehicle.
In this kind of the electrical power supply of a heating resistor the generator output voltage is regulated such that the electrical consuming devices downstream of the heating resistor are supplied with the nominal voltage level of the battery.
The disadvantage of this known arrangement is the necessity to impart to the power generator a relatively high rotary speed to achieve a voltage which would provide an adequate heating power in the series connection of the heating resistor and the power supply network which together possess relatively high impedance.
During idle operation or a slightly increased rotary speed of the generator, this arrangement cannot deliver a sufficient heating power.